Designers of manufactured consumer goods are on a continuous endeavor to reduce product weight, highly recyclable materials, environmental friendly and energy savings solutions. This has lead to usage of aluminum alloy, magnesium alloys and other ductile material selected for particular application, in respect to the material properties, behavior and application feasibilities.
In recent years, there has been an increase in magnesium alloy applications in components such as personal computers, mobile phones, home appliances, intelligent transport systems and automobile parts.
Magnesium, at one-fourth the mass of steel and two-thirds that of aluminum, fulfills the requirements of the recent trends in the industry. The characteristics of magnesium are advantageous since magnesium has good fatigue strength, dimensional stability, sound and vibration dampening qualities are very useful for variable product application. It is also feasible to manufacture products with magnesium in various shapes, forms with repeatable consistency.
However, the low ductility and deformability of magnesium die-casts can lead to slivering, layer eruption, chipped threads, and fracturing when mated with standard thread bolts or thread-cutting fasteners. Removal and reinsertion of these fasteners can weaken and even destroy threads, creating magnesium powder, debris and contamination. If a magnesium die-cast application requires a removable fastener to permit service, a special screw thread is required enable to deform the material for reliable joint, and their formation and the fastening system employed in them are required to be flawless.
However, current conventional threads are limited in terms of performance to some critical quality and functional requirement in product assembly and applications. This has lead to attempts to improve screw thread design with added reliability and characteristic such as thread forming capability in a blind and pilot hole on low ductile materials, locking action, no thread and/or boss damage, high tightening force, high stripping torque, assembly efficiency, no cracks, minimize debris creation, re-usability (repeated removal and insertion), high failure torque, high torsional strength, resistance to vibration loosening, and positive material engagement. Often conventional screws require tapping prior to fastening the screw into the material which forms a thread or grooves in the side of the hole for threads of the screw to engage. Forming such tapping holes and grooves in the side walls of the hole contribute to fracturing and other disturbances in the ductile material which contributes to the weakening of the fastening and engagement of the fastener within the ductile material.
However as mentioned, the conventional threads do not adequately meet all or some of the above critical functional requirements, which weakens the integrity of the joints. For example, some previous threads that have been disclosed include, U.S. Pat. Nos. 7,195,437, 5,061,135, and 5,340,254.
These previous screw threads have been employed in few applications on ductile materials such as magnesium and aluminum alloy, and attempt to provide sure insertion by virtue of their design, however the optimum result could not achieve in some applications such as positive engagement, failure torque, torsional strength, assembly efficiency and flexibility in product miniaturization which directly impact design solution and integrity in joints. FIG. 1A-B illustrate the limitations and some of the problems associated with the prior art. Specifically, FIG. 1A-B show a photo micrographic sectional view of a conventional screw thread, which is engaged in magnesium boss/plate 80 with pre formed hole 50 in the material where material fractures 54, insufficient of material deformation 52 and flank engagement 56 between thread crests 20 of the screw are observed.
Therefore, a thread forming screw thread and corresponding thread roll that alleviates the problems associated with the prior art is needed.